Understanding the computations that take place in brain circuits will require identifying the wiring diagrams of those circuits. In recent years seveal new methods have been developed to identify the brain's wiring diagrams. Each of these methods have some strengths and limitations. Importantly, there is no available anterograde monosynaptic tracer that can be used to regulate gene expression of synaptically connected neurons in species ranging from drosophila to mice. We propose to develop and validate a new genetically-encoded system to trace brain circuits by transsynaptic control of transcription that could overcome some of the limitations of the currently available strategies. We anticipate that this tool will open new opportunities for investigating the relationship between connectivity of neuronal circuits and brain function. The strategy that we propose is based on ligand- induced intramembrane proteolysis. In this system, neurons expressing an artificial ligand (emitter neurons) activate an engineered receptor on their synaptic partners (receiver neurons). Upon ligand-receptor interaction in synaptic sites, the engineered receptor is cleaved in its transmembrane domain and releases a protein fragment that regulates transcription in the synaptic partners. Our initial experiments in vivo, in transgenic drosophila, have confirmed the feasibility of this strategy as a method to record cell-cell interactions between neurons in the brain. We propose to optimize and validate this design towards identifying wiring diagrams of neuronal circuits in transgenic animals, both in mice and drosophila.